ASN Wikibase Occurrence # 287741
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| Date: | Thursday 24 May 2012 |
| Time: | 17:30 LT |
| Type: |  Schweizer 269C |
| Owner/operator: | Turbinator Aviation LLC |
| Registration: | N760DA |
| MSN: | S1326 |
| Year of manufacture: | 1988 |
| Total airframe hrs: | 6489 hours |
| Engine model: | Lycoming HIO-360-D1A |
| Fatalities: | Fatalities: 0 / Occupants: 2 |
| Aircraft damage: | Substantial |
| Category: | Accident |
| Location: | Piney Flats, Tennessee -
 United States of America
|
| Phase: | Approach |
| Nature: | Private |
| Departure airport: | Elizabethton, TN (0A9) |
| Piney Flats, TN | |
| Investigating agency: | NTSB |
| Confidence Rating: |  Accident investigation report completed and information captured |
The pilot reported that, while on short final for landing, he heard a "high pitched whine" and that the helicopter's tail rotor authority subsequently degraded. He immediately initiated a full autorotation to landing. During the landing in a field with uneven terrain, the helicopter was substantially damaged.
Postaccident examination revealed that, during the flight, the tail rotor driveshaft (TRDS) had become uncoupled from the drive assembly and main rotor belt transmission. The forward TRDS retention nut was found safety wired; however, it was less than hand tight. The main rotor transmission input aft pinion nut was found backed off the pinion; the nut and pieces of its fractured cotter pin were found captured in grease contained in the TRDS fitting grease cavity. The nut was intact, but it exhibited thread wear. The cotter pin was shorter and thinner than the cotter pin specified in the helicopter manufacturer's illustrated parts catalog. The grease in the aft pinion splines was discolored, caked, and dry, indicating that it had not been serviced in a long time.
The main rotor transmission input pinion exhibited significant wear on the forward spline set and the aft spline set that engaged the driving spline, which was normally retained by the aft pinion nut. The aft upper H-frame bearing was displaced on the driving spline. The driving splines' interior spline teeth exhibited severe wear matching the wear on the pinion, and rotation damage was found on the spline ends. Other areas of the pinion exhibited fretting corrosion.
According to the helicopter manufacturer's Handbook of Maintenance Instructions, during the 1,200-hour inspection, the mechanic was required to remove the upper pulley, inspect the pinion, and torque and safety wire the aft pinion nut during reassembly. The mechanic who performed the 1,200-hour inspection (and others) stated that, during the inspection, he had "never gotten into" the drive assembly, had not checked the torque on the aft pinion nut, and had not inspected the cotter pin.
Probable Cause: The mechanic's improper maintenance of the main transmission aft pinion nut and belt drive system, which resulted in the uncoupling of the tail rotor driveshaft and the subsequent loss of helicopter control.
Accident investigation:
 |
|
Sources:
NTSB ERA12LA368
Location
Revision history:
| Date/time | Contributor | Updates |
|---|---|---|
| 04-Oct-2022 13:56 | ASN Update Bot | Added |
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